Luminescent materials having a long decay period ranging from a few minutes to several hours are known and typically produce their radiation by phosphorescence. Such phosphorescent materials have typically been used in safety signs or on watch or clock dials. In recent years, technology has developed which makes it possible to imbed luminescent materials in pressed or molded plastic products. Such technology significantly broadens the range of long-decay phosphor applications.
Copper-activated zinc sulfide, such as ZnS:Cu,Cl, is frequently used for these long-decay phosphor applications because copper-activated zinc sulfide produces emission in a spectral region having a relatively high luminous efficiency. However, the properties of copper-activated zinc sulfide are not completely satisfactory since the brightness of the phosphor falls off substantially after extended decay periods, such that the emission is barely perceptible after about 30 to 60 minutes. Furthermore, copper-activated zinc sulfide is subject to degradation and deterioration when exposed to UV radiation in a moist or humid atmosphere. The body color of the material containing the zinc sulfide darkens, possibly due to the presence of elemental zinc on the surface. Use of such materials for outdoor applications has, therefore, been severely limited.
Phosphorescent materials having a long decay period may be used in such fields as the graphic arts, interior decorating or printing inks. For these applications, alkaline-earth sulfide phosphors have been used, since they can be prepared with a broad gamut of colors ranging throughout the visible spectrum from blue to red. These materials, however, are hygroscopic and react readily with moisture tending to generate hydrogen sulfide, a noxious and toxic substance. These properties severely restrict their use in the home.
A long decay phosphor having the composition of SrAl.sub.2 O.sub.4 activated with Eu.sup.2+ was reported recently (the 248th Keikoutai Dougakkai Kouen Yokou, Nov. 26, 1993). The composition of that phosphor was not fully disclosed, but it functions as a yellow-green phosphor, very similar to the emission of ZnS:Cu,Cl, and it overcomes some of the disadvantages of ZnS:Cu,Cl. The basic composition of this phosphor is not new, but was disclosed in U.S. Pat. Nos. 3,294,699 and 4,216,408. However, these patents were all directed to phosphors used for designing and making lamps.
Eu.sup.2+ may produce emission by an indirect transition over a broad spectral range wherein the emission is affected by the preparation conditions and the structure of the host crystal. For example, it is known that emission may be produced by Eu.sup.2+ all the way from the UV region to the yellow region of the spectrum depending on whether it is present in an aluminate, gallate, borate, phosphate or aluminum gallate host crystal.
Incorporation of rare earth ions in certain zinc sulfide phosphor hosts that have long decay characteristics has also been reported recently, "Study on effect of rare earth in blue-purple night-luminous phosphor CaS: Bi, Cu," Hunan Shifan Daxue, Ziran Kexue Xuebao Vol. 15, No. 2 Page 145-148, 1992 X. Mao, S. Lian and Z. Wu (Hunan Normal Univ., Hunan, CHN); and "Rare earth effect in non-radioactive night luminous phosphor ZnS:Pb,Cu," Hunan Shifan Daxue, Ziran Kexue Xuebao Vol. 14, No. 1, page 47-51, 1991, X. Mai and M. Hong, (Acta Scientiarium Naturalium Univ. Normalis Hunanensis). Such phosphors can be expected to suffer from some of the above-cited disadvantages of other sulfide-based phosphors.